1. Field of the Invention
This invention relates to hydraulic ram operated extruders for producing pieces of extrudate of substantially the same size.
2. Background Prior Art
A particular application of such apparatus is the production of blanks or "pre-forms" of rubber or similar elastomeric material for subsequent moulding in compression moulds. In such processes, it is preferable for all blanks presented to a given mould to be of exactly the same volume so as to eliminate or reduce to a minimum wastage of material, or the production of rejects in the case where undersized blanks are supplied. However rubber or rubber-like compounds have a tendency to "recover", that is shrink in the longitudinal direction after extruding with the result that the cross-sectional area may increase by up to 100% or more when leaving the extrusion die. This shrinking in length and increase in cross-sectional area may not be uniform along the length of the extrudate and thus, when cut into pieces of equal length, the pieces may be of different volume due to these variations in cross-sectional areas.
Our U.K. Patent No. 1,141,033 was directed to an improved form of hydraulic ram extruder to overcome the above difficulties by producing pieces of extrudate of substantially equal volume, the extruder having a hydraulic ram the plunger of which was arranged to apply pressure to the material to be extruded and valve means were provided for controlling and maintaining a substantially constant rate at which the fluid was supplied to the hydraulic ram. The ram had a die-shape to extrude the material in the form of a continuous length of appropriate cross-sectional shape and cutting means were arranged to sweep across the outlet of the die at equal time intervals to sever the extrudate into pieces of equal volume. Although the extruder had a valve arrangement for metering hydraulic fluid to the hydraulic ram aimed to provide fluid at a constant volume flow-rate to ensure that the ram-extruded material from the die at a constant volume flow-rate, small inaccuracies in the volume of the blanks produced still arise.
It is an object of the present invention to further improve the accuracy of blank forming in a hydraulic ram operated extruder.